Is our garden safe?

February 16, 2011 12:59 PM

Please help me decide if we should replace the pressure treated wood foundation of our raised-bed garden.

Our house came with a couple of raised-bed gardens (about 5'x5' each). The wood foundation of these beds consists of what appears to be old fence posts ... they have a green hue. There is a whole pile of these posts in the backyard that aren't being used for anything. It looks like the previous owner saw this pile and decided to make a couple of raised-beds with 8 of them. We want to start gardening this year and we're worried that the posts may be pressure treated and may contain arsenic. We have a small child and do not want to grow food that could contain increased levels of poisonous arsenic and other chemicals.

- How can we determine if the posts contain arsenic? (a little searching yielded this website, but their phone is disconnected and they haven't responded to my emails)

- If the posts test positive for CCA, should we really replace them or are we over-thinking this? At this point, wouldn't the soil already be contaminated?

I don't see any stamps on the posts. At first I thought they were just painted green (because it appears to be flaking off, but when I cut out a sample to send to the website referenced above, I saw a pretty clear green hue inside of the post.

posted by lilgoyl to home & garden (11 answers total)

Instead of testing the posts, I would test your soil. Here is a link for soil testing in King County, Washington. Soil tests are pretty commonly available, and it'll help to know that there aren't other metals that might be of concern (like lead) in your garden soil. This factsheet on gardening in arsenic and lead contaminated soils might also help you, too.
posted by ldthomps at 1:12 PM on February 16, 2011

I think it is going to be hard to figure out, if you have a good estimate of when the posts where bought that might help, most PT wood between 1975 and 2003 used CCA (Chromated copper arsenate)

Personally, I'd use them for flower beds and build new ones for the veggies.

if you are going to dispose of them, contact your local waste disposal company and ask how to do it properly, many places will not appreciate it just being dumped into the general garbage.
posted by edgeways at 1:17 PM on February 16, 2011

The green hue would be there anyway, since the replacements for CCA are also copper based and give a green tinge to the wood.

I second testing the soil. The amount of arsenic that leaches out varies based on "local climate, acidity of rain and soil, age of the wood product, and how much CCA was applied." (Source: EPA). If no detectable amounts have leached out, then I wouldn't bother replacing it, but if a significant amount has leached out you might have to replace the soil or at least only use it for flowers and other decorative plants.
posted by jedicus at 2:10 PM on February 16, 2011

It may well depend on whether the posts were new when they were made into beds (it sounds as if they weren't). The bulk of leaching occurs when new treated timber is first exposed to soil and weather. Posts that have been sitting in a pile for a year or two aren't going to leach any significant quantity of preservative into the soil.
posted by le morte de bea arthur at 3:00 PM on February 16, 2011

You may also find this article interesting- it talks about how much arsenic actually ends up in vegetables grown in CCA treated wooden raised beds; and where in the plant it ends up. By peeling even root vegetables, you can eliminate nearly all the small amounts of arsenic taken up. What seems more relevant, especially with children, is direct exposure to CCA treated wood (it can be sealed with penetrating oil sealers, BTW).

Rufus Chaney at the USDA agrees with Bourquin about food safety. “There’s no evidence that food safety is impaired by growing vegetables around CCA-treated wood.” According to Chaney, high levels of inorganic arsenic in soil will kill a plant before there’s enough arsenic in the plant itself for you to consider not eating it. Far more important is the risk of potential transfer of arsenic to skin and mouths, particularly for children, whose small bodies don’t tolerate arsenic as well as ours do. Chaney points out that persistent leaching, however small, means that arsenic is continually coming to the surface of the wood, where it can easily be transferred to us or our children when we touch the wood. “There’s just no way around it,” Chaney says. “For me, this is the overriding reason not to use CCA.”
posted by oneirodynia at 4:39 PM on February 16, 2011

Here are pictures of the garden in question. I've called several local extension offices, the DEQ, etc, and they just refer me to mail-in labs. No one seems to offer free testing any more. Looks like I'll have to shell out around $30 bucks for an arsenic test.
posted by lilgoyl at 4:45 PM on February 16, 2011

Those barely look like raised beds. If I were you, I would properly dispose of those posts so you don't have to worry about contact toxicity in the case of your child ( a much more significant source of ingestion than vegetables grown in soil with CCA leachates). Then build some proper raised beds of at least 18" deep, and fill with new, clean soil. If you want to really minimize any take up of toxins, read ldthomp's factsheet in their second link for how to do this.
posted by oneirodynia at 4:59 PM on February 16, 2011

Those barely look like raised beds. I second this. They almost look like they are there for some kind of ornamentation. If you want real raised beds, you'll have to build them.
posted by carter at 5:47 PM on February 16, 2011

To arrange soil testing, contact the Cooperative Extension Service.
posted by theora55 at 8:20 PM on February 16, 2011

Soil sample is in the mail folks. No state/county agencies were willing/able to test; they all referred me to labs. After calling a few labs, I finally talked to a tech who was able to answer a lot of my questions and I'm sending him a sample for a total arsenic test at a cost of $26.25 (plus postage).

I took a sample of the soil directly beneath one of the posts, as I assume that would have the highest concentration of arsenic (if any). If the test shows an increased level of arsenic in the soil, I will rebuild the beds with new lumber and soil.

If the arsenic levels fall within the acceptable range for this region, I'll likely add some more posts from the pile (to raise bed height) and add some more soil.

I tend to agree that the majority of hazardous chemicals probably already leached out of these poles when they were fence poles long ago, so they're probably safe for use now .... but I'll let the expert tell me that for sure ;)

The suggestion to test the soil rather than the post itself is what really helped. Thanks!
posted by lilgoyl at 10:34 AM on February 17, 2011

We received the results from our soil test this weekend. The amount of arsenic in the garden soil was measured at 261 ppm. For reference, the national average is 5.2 ppm.

The soil in our garden contains 5,000% more arsenic than the national average - I think it's a safe assumption to say that the posts are still pretty darn toxic ;)

We'll be disposing of the posts and a good amount of the soil that's currently there. Then we'll be building some brand new 24" beds and filling with new soil.

I may post another question about this but feel free to provide me with input if you have any ... I'll be building the raised beds from premium doug fir and am considering coating the wood with Thompson's Water Seal (oil based). Do you think this is safe for the new vegetable beds?
posted by lilgoyl at 11:59 AM on March 7, 2011